[dm-devel] snapshot-origin freezes system - what am I doing wrong?

[Zdenek Kabelac]

Dne 15.5.2015 v 18:47 Atom2 napsal(a):
> Am 15.05.15 um 14:11 schrieb Zdenek Kabelac:
>> Dne 15.5.2015 v 12:45 Atom2 napsal(a):
>>> Hello list,
>>> I am trying to setup a cow snapshot for a LV that is used as a master image
>>> for a number of VMs. The idea basically is to be able to update the master
>>> image even when VMs are up and running; the VMs should then still see the old
>>> state of the image and only when they are restarted they should connect to the
>>> new image.
>>> Searching the net seemed to point towards a snapshot-origin/snapshot solution
>>> - however I am unable to get this to work. Information on the net seems to be
>>> sparse, so I though I'd ask the experts on the list. Here are my steps:
>>>
>>> 1.) I have a LV in volume group VG named master.ROOT
>>> (/dev/mapper/VG-master.ROOT), 8GB, formatted as ext4
>>> 2.) I create a sparese file: truncate -size=8G /tmp/snapshot
>>> 3.) losetup -f /tmp/snapshot --> gives /dev/loop0
>>> 4.) dmsetup create mytest.img --table "0 $(blockdev --getsz
>>> /dev/mapper/VG-master.ROOT) snapshot-origin /dev/mapper/VG-master.ROOT
>>> 5.) dmsetup create mytest.img.cow --table "0 $(blockdev --getsz /dev/loop0)
>>> snapshot /dev/mapper/VG-master.ROOT /dev/loop0 P 8"
>>>
>>> So far so good ... however, when I try to mount the origin device by
>>> 6.) mount /dev/mapper/mytest.img
>>> the mount call doesn't return and the system gets unresponsive/freezes up to a
>>> point when OOM-killer is being invoked. Login attempts on the console time out
>>> and in essence it is only possible to reboot the system using magic-sysreq key
>>> combinations.
>>>
>>> I'd be very much obliged if someone in the know could provide me with
>>> information what's wrong with this approach.
>>>
>>> Many thanks in advance Atom2
>>
>> Do you have any  scientific reason to not  use LVM2  here ?
> As far as I understand the snapshot-origin target with a cow snapshot is not
> (yet) directly supported by LVM2.

Just curious - from where have you got this idea ??

>
> Therefore I have so far worked from an assumption that I need to use dmsetup
> directly to solve my use case. If, however, you tell me otherwise and are able
> to show me how to use LVM2 instead, I am more than happy to go down that
> route. Let me once again outline my use case:
>
> I have a LV (that's the master image/template which was actually setup with
> LVM2) which is maintained from within a template VM and serves as the (r/o)
> root image for a number of other (dependent) virtual machines. All dependent
> virtual machines mount that template LV r/o and overlay it with an overlayfs
> as a r/w layer for write access.
> I now want to be able to update the master image from within the template VM
> and be able to update it at any time; running dependent VMs should however not
> see any changes to the image while they are up, hence the requirement to have
> a snapshot-origin target and a snapshot that would cow buffer any changes in
> the template (origin) target until such time the dependent VM is restarted. At
> that point in time the dependent VM would (if required setup and) connect to a
> new snapshot.
>
> I hope that clarifies my setup.
>
> Furthermore, IMHO and in any case mounting a dm-target should under no
> circumstances in essence bring down a machine - even if something is horribly
> wrong.
>
>> Management of snapshot target is not trivial - especially the order
>> of individual table loads and resumes.
>>
>> You could look at  'lvcreate -s -vvvv' if you are interested in ioctl
>> ordering of all operations here.
>>
> I am probably missing something here, but my steps listed above do not involve
> lvcreate ... so how could I use that -vvv?
>

Well even kernel documentation for the snapshot target itself is rather 
referencing usage of snapshot via lvm2 - and it's for a reason.

https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt

So if you could use  lvm2 - stay with lvm2 and avoid your home-brew dmsetup 
commands - it needs deep understanding how the old snapshot target works:

http://people.redhat.com/agk/talks/LVM2-LinuxTag2006/


Regards

Zdenek